Evolution of the Paleo Tethys From the Devonian Through Permian

Devonian to Permian evolution of the Paleo-Tethys Ocean: New evidence from U-Pb zircon dating and Sr-Nd-Pb isotopes of the Darrehanjir-Mashhad “ophiolites”, NE Iran

Authors:

Moghadam et al

Abstract:

Middle to Late Paleozoic ophiolites, which are remnants of the Paleo-Tethys Ocean, are aligned in two main zones in northern Iran: Darrehanjir-Fariman-Mashhad, and Rasht in the north and Jandagh-Anarak ophiolites to the south. Our new U-Pb zircon dating results shows that the ~ 200km long Darrehanjir – Mashhad mafic-ultramafic body is not a single ophiolite but a composite igneous complex composed of Permian pillow lavas and pelagic sediments in fault contact with a small outcrop of Devonian intrusive and ultramafic rocks. Darrehanjir intrusive rocks have U-Pb zircon ages of 380.6 ± 3.7 Ma and 382.9±3.7 Ma respectively, ~ 100 Ma older than published ages for gabbros and radiolarites intercalated with lavas near Mashhad and Fariman. Mantle peridotites from the Devonian complex contain low Cr# spinel, similar to that in MORB-type peridotites. Devonian Darrehanjir gabbros and Permian Mashhad sequences both have boninitic and calc-alkaline signatures, respectively. The δ18Ozircon values from the Devonian ferrodiorite (δ18Ozircon~ 4.6±0.3‰) are slightly lower than the 5.2 to 5.4‰ expected for MORB-type zircons whereas Devonian plagiogranitic zircons mostly have δ18Ozircon < 5‰, perhaps reflecting involvement of hydrothermally altered crust. Similar, strongly positive values of zircon εHf(t) for plagiogranite (av. + 14.9) and ferrodiorite (av. + 13.8) indicate melt derivation from depleted asthenosphere. Darrehanjir-Mashhad ophiolitic rocks can be divided into groups with high εNd (> 10.3) and low εNd (< 5.4) for both Permian and Devonian suites. Most Darrehanjir-Mashhad rocks are characterized by radiogenic 207Pb/204Pb and 208Pb/204Pb, indicating the involvement of subducted terrigenous sediments in the source. The Mashhad-Darrehanjir mafic-ultramafic complex demonstrates that this part of Paleo-Tethys evolved from oceanic crust formation above a subduction zone in Devonian time to accretionary convergence in Permian time. Iranian Paleozoic ophiolites and oceanic igneous complexes along with those of the Caucausus and Turkey in the west and Afghanistan, Turkmenistan and Tibet to the east, define a series of diachronous subduction-related marginal basins that were active from at least Early Devonian to Late Permian time.